This invention relates to novel solutions useful in the production of fire resistant glazings comprising an alkali metal aluminate and a silicate waterglass, to methods for the preparation of those solutions and to the production of intumescent interlayers from those solutions which may be incorporated into fire resistant glazings.
Glass laminates incorporating an intumescent inorganic silicate layer sandwiched between two opposed panes of glass are sold under the trade marks PYROSTOP and PYRODUR by the Pilkington group of companies. When such laminates are exposed to a fire the inorganic layer intumesces and expands to form a foam. The foam provides a thermally insulating layer which protects the pane of glass remote from the fire so that the structural integrity of the glass unit and thereby a barrier to the propagation of the fire is maintained for a longer period. Glass laminates incorporating such intumescent layers have been used successfully as fire resistant glass structures. These laminates may comprise more than two panes of glass sandwiching more than one intumescent interlayer. Laminates comprising up to eight intumescent layers have been employed. These multi layered laminates are relatively thick and correspondingly expensive.
The intumescent inorganic layer is normally formed from a sodium silicate waterglass or a mixture thereof with a potassium silicate waterglass. The layer is commonly formed by preparing a solution of the waterglass (or waterglasses), spreading that solution on the surface of the glass and drying excess water from the solution so as to form the intumescent inorganic layer.
U.S. Pat. No. 4,190,698 discloses fire resistant glazings comprising an intumescent inorganic layer obtained by drying a waterglass solution. The authors suggest the addition of various additives to the waterglass solution including urea, polyhydric alcohols, monosaccharides, polysaccharides, sodium phosphate, sodium aluminate, borax, boric acid and colloidal silica. There is no specific disclosure of the addition of an aluminate to a waterglass solution.
We have discovered that the addition of sodium aluminate to a waterglass solution as proposed in U.S. Pat. No. 4,190,698 does not produce a solution which is useful in the production of fire resistant glazings. The solutions are unstable and form a precipitate immediately or on standing. Since the dried interlayer must be optically clear the presence of particulate material such as this precipitate is not acceptable.
Applicants have discovered that it is possible to produce a clear stable solution comprising an aluminate and a waterglass which can be used in the production of fire resistant glazings by partially neutralising the aluminate with a hydroxy carboxylic acid prior to mixing it with the waterglass. Thus from one aspect this invention provides a clear stable solution useful in the production of fire resistant glazings comprising an alkali metal silicate waterglass, a water soluble aluminate and a hydroxy caboxylic acid.
The water soluble aluminate is preferably an alkali metal aluminate such as lithium aluminate, potassium aluminate, caesium aluminate and most preferably sodium aluminate. Other water soluble aluminates notably ammonium aluminate and alkyl ammonium aluminates may also be employed.
The carboxylic acid is preferably a hydroxy carboxylic acid and more preferably an α-hydroxy carboxylic acid. Examples of preferred carboxylic acids include tartaric acid, malic acid, gluconic acid, lactic acid, saccharic acid and most preferably citric acid.
The waterglasses useful in the compositions of this invention are preferably sodium silicate waterglasses. The preferred sodium silicate waterglasses are those wherein the weight ratio of SiO2:Na2O is at least 2.0:1 more preferably at least 2.5:1 and most preferably at least 2.85:1. Sodium silicate waterglass solutions wherein the weight ratio of SiO2:Na2O varies between 2.0:1 and 4.0:1 are available as articles of commerce. Specifically solutions wherein this ratio is 2.0:1, 2.5:1 and 2.85:1 3.0:1 and 3.3:1 are available as articles of commerce. Waterglasses having any particular weight ratio of SiO2:Na2O may be produced by blending these commercially available waterglass solutions. Sodium silicate waterglasses having a weight ratio of SiO2:Na2O of 2.0:1, 2.5:1 and 2.85:1 are preferred for use in the present invention.
Potassium silicate and lithium silicate waterglasses may also be used in the compositions of this invention. In a preferred embodiment these waterglasses will be used as a partial replacement for the sodium silicate waterglasses. In general the molar ratio of sodium to potassium and/or lithium in the waterglass solution will be at least 2:1.
In particular preferred embodiment the waterglass solution used in the present invention comprise a mixture of sodium silicate waterglass and a potassium silicate waterglass. The molar ratio of sodium ions to potassium ions in these mixtures in preferably at least 4:1. Where a potassium silicate waterglass is employed it is preferably one where the weight ratio SiO2:K2O is in the range 1.43:1 to 2.05:1.
The solutions of this invention may further comprise one or more polyhydric compounds which are known to be useful in existing intumescent interlayers. Polyhydric compounds which have been proposed for this use include glycerol, glycerine or a derivative of glycerine or a sugar. The most commonly used polyhydric compound and the preferred polyhydric compound for present use is glycerol.
In order to be useful in the production of intumescent interlayers the solution of this invention should preferably be clear, stable and capable of being dried to form a useful intumescent interlayer. The properties of the solution are affected by the composition of the solution and the methods used in their preparation. The utility of any particular solution may be determined by experiment. The stability of the solutions decreases as the amount of aluminate present increases. However the fire resistance of the interlayer increases in proportion to the amount of aluminate present. The amount of aluminate which is incorporated into the solution will preferably represent a compromise between these two properties and will typically be such that the molar ratio of silicon to aluminium is in the range 20:1 to 35:1 more preferably in the rage 25:1 to 32:1.
A second significant factor which affects the utility of the solutions of this invention is the weight ratio of silica to alkali metal oxide. Increasing the proportion of silica reduces the stability of the solution and is undesirable. The lower ratios of silicon to alkali metal are preferred since this increases the flow point of the dried interlayer which is an important factor influencing the performance of a glazing incorporating such an interlayer in fire testing. Generally we prefer that the weight ratio of silica to alkali metal oxide is in the range 2:1 to 4:1.
The solutions of the present invention must be clear. They are prepared by mixing the various components in a manner which produces a clear stable solution which may then be dried to form a clear stable intumescent layer.
We prefer that the solutions are prepared using a process which comprises as a first step partially neutralising the aluminate with the hydroxy carboxylic acid. This neutralisation may be carried out by adding a hydroxycarboxylic acid solution to an aqueous solution of the aluminate. As the presence of excessive quantities of water may destabilise the silicate solutions of the invention and because in any event excess water is an additional load on the drying process used to form the intumescent interlayer the aluminate solution will preferably contain as high a solids content as is compatible with the production of a clear stable solution. The aluminate solution may typically comprise from 20 to 45% by weight of solids.
The hydroxy carboxylic acid solution may be added to the aluminate either as a solid or as a solution. The solution may be an aqueous solution. In the preferred embodiments where the solutions of this invention comprise a polyhydroxy compound which is preferably glycerol the hydroxy carboxylic acid may be dissolved in the polyhydroxy compound and the resulting solution is used to neutralise the aluminate.
The addition of the acid solution to the aluminate solution is preferably continued until the solution has a pH in the range 9.0 to 11.0 and more preferably in the range 9.5 to 10.0. The neutralisation should be carried out with thorough mixing and in a manner such that the temperature of the reaction mixture is not allowed to rise excessively and is preferably maintained below 50° C.
Solutions which comprise a water soluble aluminate, a hydroxy carboxylic acid and a polyhydric compound which is preferably glycerol are believed to be novel and comprise a further aspect of the invention.
The partially neutralised aluminate solution may be added to a solution comprising an alkali metal silicate waterglass to form a solution according to the present invention. The mixing of the aluminate with the waterglass must be carried out in a controlled manner so as to avoid the formation of any precipitate. We prefer to add the aluminate to the waterglass at a controlled rate with thorough mixing.
The resulting solutions may be used in the production of five resistant glazings using known techniques. In one existing process the solutions may be spread upon the surface of a glass sheet which is provided with an edge barrier which retains the solution on the surface of the glass. The quantity of solution employed will vary with the desired thickness of the dried interlayer. The quantity needed to produce an interlayer of any particular thickness may be determined by routine experiment.
The solution is then dried under carefully controlled conditions of temperature and humidity so as to ensure the production of a clear transparent interlayer which is free from bubbles and other optical imperfections. The dried interlayers generally comprise form 10 to 35% by weight of water. The aluminium content of the dried interlayer is generally in the range 0.1% to 5.0% by weight preferably from 0.1% to 1.0% by weight. We have discovered that the presence of the aluminium improves the performance of glazings into which the interlayer is incorporated in terms of their fire resistance and their mechanical impact resistance properties.
The thickness of the dried interlayer will generally be in the range 0.1 to 2.0 mm preferably from 0.5 to 2.0 mm. The formations of thicker interlayers requires a longer drying time and is thereby disadvantageous. Thinner interlayers can be produced using shorter drying times. A glass laminate having a thicker interlayer may be produced by bringing two sheets of glass each having a relatively thin interlayer having say a thickness of from 0.5 mm to 1.0 mm into face to face contact so as to form a laminate having an intumescent interlayer which is from 0.2 to 2.0 mm thick.
Flat glass sheets of various thickness may be used in the laminates of the present invention. Typically sheets of soda lime float glass having a thickness of from 2.0 mm to 4.0 mm are employed.
The edge barrier is normally cut away at the completion of the drying process to leave a glass sheet having a dried interlayer on one surface. A laminate may be formed by placing a second glass sheet on top of the interlayer. In another embodiment the second glass sheet may itself be one having an intumescent interlayer on one surface thereof. Mounting this second sheet on top of the first sheet so that the two fire resistant interlayers are in contact with each other produces a laminate having a relatively thick interlayer. Mounting the second sheet so that the interlayer is on its upper surface and subsequently providing a third glass sheet on top of that second interlayer produces a laminate having two interlayers mounted between three panes of glass. Laminates having as many as eight interlayers may be produced.
In an alternative process the solutions of this invention may be poured onto the surface of the substrate and dried to form an intumescent fire resistant interlayer which is sufficiently strong to be removed from the substrate in the form of a transparent film. The film may then be placed between two sheets of glass to form a fire resistant glass laminate. A variety of substrates may be employed in processes of this type examples being glass sheets, metal sheets and polymeric materials such as PTFE and polyolefins such as polypropylene.
The invention is illustrated by the following example